Human immunodeficiency virus (HIV-1) is the major cause of acquired immunodeficiency syndrome (AIDS) in humans. The HIV- 1 envelope glycoproteins, gp120 and gp41, are contained in trimeric complexes on the virion surface. The gp120 envelope glycoprotein mediates virus attachment to target cell receptors, CD4 and the chemokine receptors, CCR5 or CXCR4. Receptor binding triggers conformational changes in the HIV-1 envelope glycoproteins leading to the fusion of the viral and target cell membranes, a process mediated by the gp41 transmembrane envelope glycoprotein. During natural infection, envelope glycoprotein- receptor interactions can potentially be interrupted by neutralizing antibodies directed against the gp120 glycoprotein or by chemokines. In addition, a number of small-molecular- weight inhibitors that bind to the chemokine receptors have been developed and some of these may proceed to clinical trials. In this application, a series of studies are proposed that will extend our knowledge of the interactions of chemokine receptors with HIV-1 gp120 and with the chemokines. In Specific Aim 1, the CXCR4-binding sites on the gp120 glycoproteins of X4 and R5X4 HIV-1 isolates will be mapped, thus allowing comparison of the binding sites for CXCR4 and CCR5. In Specific Aim 2, the mechanisms by which HIV-1 adapts to utilize wild-type and variant chemokine receptors in a CD4-independent fashion will be explored. In Specific Aim 3, the mechanism of HIV-1 adaptation to drugs directed against CCR5 will be investigated. In Specific Aim 4, the contribution of O-linked glycosylation to CCR5 function will be explored. Together, these studies will provide a comprehensive picture of the interactions of the CCR5 and CXCR4 chemokine receptors with key ligands.